Combustion of LPG in a Spark-Ignition Engine

Tax concessions promote the use of Liquefied Petroleum Gas (LPG) fuel for automotive use in Europe. Modelling of the LPG evaporation process shows the importance of drawing the liquid from the tank rather than the gas, otherwise the most volatile component (propane) is used more quickly and the composition of the remaining fuel changes. It is shown that the LPG components have similar calorific values to gasoline, however injecting the LPG as a gas into the inlet port causes a loss of volumetric efficiency and peak power.

The experimental results showed:

• The LPG fuels have similar burn rates and optimum ignition timing to gasoline.
• The Lean Mixture Limit (LML) of the gaseous fuels was weaker than that for gasoline.
• N-butane showed a higher Knock Limited Spark Advance (KLSA) than gasoline, iso-butane showed only slight knock at a very high spark advance, and propane did not cause knock under any test conditions
• The knock limit test used was a more realistic test than the tests used to find RON and MON values. The tests showed that MON was a better indicator of the anti-knock characteristics of fuel than RON.

The study found that the components of LPG fuel have similar characteristics to gasoline with similar burn rates and optimum ignition timing, but extended lean mixture limits.